1. Field of the Invention
The present invention relates to measurement instruments, and, more particularly, to a method and apparatus for measuring borehole characteristics.
2. Description of the Related Art
Resistivity logging tools are instruments used in operations in boreholes that are drilled into underground rock formations in the search for oil, gas, or minerals. Resistivity logging tools measure the electrical conductivity of the rock formations accessible from the borehole, and more particularly, perform multiple simultaneous measurements while drilling, at each of several different radial distances from the axis of the borehole. Using the measurements taken by resistivity logging tools, a number of samples of rock resistivity can be combined into an image log as the tool progresses up the borehole. The image logs can make apparent variations or differences in rock resistivity due to invasion of the rock by drilling mud filtrate. Analysts can recognize valuable information about the constituents of the rock and other useful information, and automated software algorithms can also extract information about the amounts and producability of hydrocarbon deposits.
Existing tools for the purposes described above typically use the induction principle, with transmitting coils and receiving coils, at frequencies in the range of 200 KHz to 2 MHz. Although commonly referred to as “Wave Propagation Tools,” the common reference is a misnomer because, at these frequencies, dimensions, and rock conductivities, the electromagnetic wave is in a degenerate mode dominated by skin effect. Measurements are commonly made of the phase-shift and attenuation of the signals at the receiver coils, which are indicative of the rock conductivity.
Maxwell's equations and known properties of the phenomenon of “skin-effect” in conductive media show that the phase and amplitude of an electromagnetic field established in a rock formation are altered by the properties and boundaries between differing regions of the rock in complex ways. A simple wave propagation resistivity tool includes a transmitter coil and a receiver coil. An electromagnetic field is created by current flowing in a transmitter coil placed coaxially in the borehole. The electromagnetic field propagates through the rock and is sensed by one or more receiver coils disposed coaxially with the transmitter coil, but spaced apart from the transmitter coil. Useful properties of the sensed signals include the phase and amplitude of the sinusoidal signals induced in the receiver coils.
The phase and amplitude differences of the voltages in adjacent receiver coils are representative of the components of the electromagnetic field coupled to the receiver coils after passing through the rock medium more distant from the borehole. Using the differences provides a compensation system for sensing the subtle variations in rock conductivity at a radial distance from the axis of the borehole. The actual distance measured typically depends on the transmitter-receiver spacing employed.
Often a borehole contains highly conductive mud, and the amount of the highly conductive mud increases in quantity when the diameter of a borehole increases after a washout of softer rock in the borehole. The increase in diameter can introduce errors in measurements due to additional local phase shift in, for example, one of the two receivers.
A useful technique to correct this problem is known as “Borehole Compensation,” whereby at least two transmitter coils are used with each pair of receiving coils, the transmitters being positioned above and below the receivers and energized alternately. When the successive sets of data from receivers are combined, the effect of borehole diameter variations (and mismatched receiver circuits) is cancelled out.
The major problem with existing borehole compensation methods is the need to alternately energize two transmitters. Alternate energizing enables the task of measuring the two phase difference signals when the two transmitter coils operate at identical frequencies. However, using two transmitter coils alternately leads to errors in the form of incomplete compensation of borehole caving due to the time delay between sequential measurements, and the problem is amplified when a measurement tool is moving at high speed. Moreover, the incomplete compensation problem is compounded when there are multiple transmitters for different radial depths of investigation. For example, the multiple transmitter signals often need to be time-multiplexed when operating at the same frequency to avoid cross-talk. The problems caused by alternate energizing of coils leads to significant slowing of the rate of data acquisition. Further, the alternate energizing leads to errors in the form of incomplete compensation of borehole caving due to the time delay between sequential measurements and lowered signal to noise ratios due to the starting and stopping (i.e., duty cycle) of the transmitters. Errors are magnified by the time delays when drilling rates are high. Also, the multiplexing slows the rate of data acquisition.
The prior art provides for methods of acquiring data via measuring the attenuation of the amplitude of the waves. The amplitude is expressed as a logarithm of the received voltage signal, often expressed in units of decibels (dB), which has a particular use in gaining an understanding of rock resistivity. A description of these methods and of electronic circuits to perform one borehole compensation method are provided in U.S. Pat. No. 5,428,293 “Logging While Drilling Apparatus with Multiple Depth of Resistivity Investigation,” to Paul L. Sinclair, and assigned to Halliburton Logging Services Inc. Although it has long been recognized that it would be most advantageous to be able to make simultaneous signal transmissions for borehole compensation, the problem of separating the upward and downward signals remains.
Therefore, there is a need for a borehole compensation method and system that avoids the requirement of alternately energizing transmitter coils and allows for faster rates of data acquisition.